fix: safe type assertions in CloseNotify() and Hijack()

[jassus213]

Problem

Similar to #4460 (fixed for Flush() in #4479), Hijack() and CloseNotify() perform direct type assertions that panic when the underlying writer doesn't implement the interface.

When gin is used with http.TimeoutHandler or any middleware that wraps http.ResponseWriter with a type that doesn't implement http.Hijacker or http.CloseNotifier, calling Hijack() or CloseNotify() causes a runtime panic:

// panics if underlying writer is not http.CloseNotifier                                                                                                                                    
return w.ResponseWriter.(http.CloseNotifier).CloseNotify()                       
                                                          
// panics if underlying writer is not http.Hijacker       
return w.ResponseWriter.(http.Hijacker).Hijack()                                                                                                                                            

Fix

Replace direct assertions with checked assertions - the same pattern already used in Flush():

• CloseNotify() returns nil when unsupported (http.CloseNotifier is deprecated since Go 1.11; callers should use Request.Context().Done())
• Hijack() returns errHijackNotSupported, consistent with the existing errHijackAlreadyWritten pattern

The capability check in Hijack() now runs before mutating w.size, so Written() stays false on the error path.

Tests

Added TestResponseWriterOptionalInterfaceFallbacks with a mockNonHijackerWriter (implements only http.ResponseWriter) verifying that Flush(), CloseNotify(), and Hijack() all degrade gracefully without panicking.

Closes #4638.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.43%. Comparing base (3dc1cd6) to head (1f5552f).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4639      +/-   ##
==========================================
- Coverage   99.21%   98.43%   -0.79%     
==========================================
  Files          42       48       +6     
  Lines        3182     3761     +579     
==========================================
+ Hits         3157     3702     +545     
- Misses         17       50      +33     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.42% <100.00%> (?)
-tags go_json	98.36% <100.00%> (?)
-tags nomsgpack	98.41% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.43% <100.00%> (?)
go-1.26	98.43% <100.00%> (?)
macos-latest	98.43% <100.00%> (-0.79%)	⬇️
ubuntu-latest	98.43% <100.00%> (-0.79%)	⬇️

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